player: improved load_png_texture() when libpng is disabled
player: improved load_png_texture() when libpng is disabled

Improve warning message, making it clearer that execution will
continue, but image is not loaded. Also fix compiler warnings
on unused arguments.

Signed-off-by: Julien Olivain <7fdfe229fce69a4d7f38653f3755ccbae5703f88@cotds.org>

jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player